There is an increasing demand for reduction of power consumption (improvement in power efficiency) of power amplifiers for use in mobile phone base stations or the like. This is because the reduction in power consumption obviously provides an effect of saving electricity costs and reduction in energy consumption provides an effect of reducing environmental load. Not only these effects, the amount of heat generated by the power amplifier can also be reduced, which enables reduction of surface area of a radiator plate required for dissipation of heat, and hence reduction of volume of the power amplifier.
In order to improve the efficiency of power amplifiers, Doherty amplifiers are commonly used (see, for example, JP-A-2006-166141 (Patent Document 1)). A Doherty amplifier comprises a carrier amplifier which constantly performs signal amplification operation, and a peak amplifier which operates only during high power output. The Doherty amplifier is configured to divide an input signal into the carrier amplifier and the peak amplifier, and to combine outputs of the carrier amplifier and peak amplifier (see, for example, JP-T-2008-535321 (Patent Document 2)).
FIG. 1 is a block diagram illustrating a configuration of a common Doherty amplifier. The Doherty amplifier comprises a carrier amplifier 1, a peak amplifier 2, an input power divider circuit 3, and an output power combiner circuit 4. Each of the carrier amplifier 1 and the peak amplifier 2 includes a field-effect transistor (FET) or the like.
FIG. 2 is a block diagram illustrating an example of a configuration of the carrier amplifier 1 or the peak amplifier 2 when a FET is used. The carrier amplifier 1 (or the peak amplifier 2) comprises a FET 5, a DC coupling capacitor 6, an input matching circuit 7, an output matching circuit 8, a RF choke coil 9, a by-pass capacitor 10, a DC power supply 11, and so on.
In general, the carrier amplifier 1 is biased to class AB or class B, while the peak amplifier 2 is biased to class C. By providing the carrier amplifier 1 operating near a saturation output power level while maintaining the saturation, a higher efficiency than normal class-A or class-AB amplifiers can be realized even during output with a back-off from the saturation output power.
FIG. 3 is a diagram illustrating efficiency characteristics versus output power of a power amplifier employing the Doherty amplifier configuration shown in FIG. 1. In FIG. 3, the horizontal axis represents output signal power, while the vertical axis represents efficiency. The efficiency can be represented by the following equation.Efficiency=(output signal power/applied DC power)×100[%]
It is assumed here that the carrier amplifier 1 and the peak amplifier 2 have the same saturation output level. In this case, the power amplifier exhibits efficiency peaks, with respect to the combined saturation power of the carrier amplifier 1 and the peak amplifier 2, at the 6 dB back-off point (peak point on the left side (lower output power side) in FIG. 3) where the output of the carrier amplifier 1 is saturated, and at the 0 dB back-off point (peak point on the right side (higher output power side) in FIG. 3) where the output of the peak amplifier 2 is also saturated.
Herein, a Doherty amplifier, in which devices having the same saturation output level are used as the carrier amplifier 1 and peak amplifier 2, and which exhibits efficiency peaks at the 0 dB back-off point (the peak point on the right side (lower output power side) in FIG. 3) and at the 6 dB back-off point (the peak point on the left side (higher output power) in FIG. 3) from saturation power, shall be referred to as the “symmetric Doherty amplifier”.
When the saturation output level of the carrier amplifier 1 is denoted by X [W], and the saturation output level of the peak amplifier 2 is denoted by Y [W], (X/Y) is called power ratio. Therefore, the “symmetric Doherty amplifier” has a power ratio (X/Y) that is constantly equal to one. However, it is usually impossible in an actual circuit that the power ratio (X/Y) is exactly one. Accordingly, it can be said that the power ratio (X/Y) of the “symmetric Doherty amplifier” is substantially equal to one.
Further, a technique is also commonly known in which highly efficient operation of a Doherty amplifier is made possible at an arbitrary operating point by forming the carrier amplifier 1 and the peak amplifier 2 by devices having different saturation output levels so as to vary the peak point (see “RF POWER AMPLIFIERS FOR WIRELESS COMMUNICATIONS”, Steve C. Cripps, ARTECH HOUSE MICROWAVE LIBRARY, April, 1999). Such a Doherty amplifier shall be herein referred to as the “asymmetric Doherty amplifier”.
The “asymmetric Doherty amplifier” has a power ratio (X/Y) that is not equal to one.
FIG. 4 is a diagram illustrating comparison of efficiency characteristics versus output power between the symmetric Doherty amplifier and the asymmetric Doherty amplifier. In FIG. 4, the asymmetric Doherty amplifier is illustrated by way of example as having an efficiency peak at a back-off point greater than 6 dB (peak point on the left side (lower output power side) of the dashed line in FIG. 4) on the assumption that [saturation output level Y of the peak amplifier 2]>[saturation output level X of the carrier amplifier 1]. For example, when a device having a saturation output level Y of 90 W is used as the peak amplifier 2 while a device having a saturation output level X of 30 W is used as the carrier amplifier 1 so that the power ratio (X/Y) is (1/3), the asymmetric Doherty amplifier will have an efficiency peak at 12 dB back-off point.
JP-A-2007-081800 (Patent Document 3) discloses an example of an asymmetric Doherty amplifier configuration in which semiconductor devices having the same or different saturation output levels are used as a carrier amplifier and a peak amplifier, and different supply voltages are respectively supplied to the carrier amplifier and the peak amplifier so that they have different saturation output levels.
In power amplifiers, in general, a necessary number of amplifiers are cascaded in order to satisfy a gain requirement.
FIG. 5 is a block diagram showing an example of such a power amplifier 20′. The illustrated power amplifier 20′ has a configuration in which a driver stage amplifier 13′ is cascaded to a final stage amplifier 14 that is configured as the aforementioned Doherty amplifier. In comparison with the final stage amplifier 14, the driver stage amplifier 13′ has a lower output power level and hence has lower power consumption. Therefore, the driver stage amplifier 13′ is usually formed by a class-A or class-AB amplifier 12 having a simple circuit configuration.
FIG. 6 is a graph in which efficiency characteristics are plotted against output power for the driver stage amplifier 13′, the final stage amplifier 14, and the power amplifier 20′. Since the Doherty amplifier configuration is not used for the driver stage amplifier 13′, the relationship of its output signal power versus efficiency characteristics is represented by a straight line as shown by the two-dot-chain line in the graph. The relationship of output signal power versus efficiency characteristics of the final stage amplifier 14 that is a Doherty amplifier (represented by the dashed line in the graph) is added to this linear relationship, whereby the relationship of output signal power versus efficiency characteristics of the power amplifier 20′ can be obtained as represented by the solid line.
On the other hand, a mobile communications system is a system in which an amount of communication traffic varies depending on time and location. One of such mobile communications systems is a W-CDMA (Wideband Code Division Multiple Access) modulation wave system.
The W-CDMA modulation wave system, having a large amount of communication traffic, is sometimes operated with four carriers as shown in FIG. 7A, whereas it is sometimes operated with a single carrier as shown in FIG. 7B especially when the amount of communication traffic is small such as during night time.
In the W-CDMA modulation wave system, a signal power level of each carrier also varies from moment to moment. For example, when the system has a constant power level per carrier, a difference in signal power level is as great as 6 dB between when the system is operated with four carriers and when operated with a single carrier. It is assumed that the system is designed by using a circuit configuration of a Doherty amplifier such that a maximum efficiency is obtained when the system is operated with four carriers. In this case as well, the power amplifier cannot be operated at a high efficiency when the system is operated with a single carrier, that is, when the signal power level is lower by 6 dB than when the system is operated with four carriers.
Therefore, in the field of power amplifiers for use in a system in which transmission power varies depending on a magnitude of amount of communication traffic, it is desired to develop a power amplifier capable of operating at higher efficiency than conventional ones both in a high transmission power level region and in a low transmission power level region.
As mentioned in the above, a circuit configuration of a Doherty amplifier is employed for enhancing the efficiency of power amplifiers. In a typical Doherty amplifier configuration, the maximum peak of efficiency is found at the back-off points of 0 dB and 6 dB from the saturation output power level. The technique has also been established in which devices having different saturation output levels are used as a carrier amplifier and a peak amplifier so that the Doherty amplifier is enabled to operate to exhibit its maximum efficiency point at a desired operating point by changing the efficiency peak point of the Doherty amplifier from 6 dB. When this technique is applied to a power amplifier which amplifies a modulation signal having a peak factor as large as 7 dB to 11 dB such as W-CDMA modulated waves or OFDMA (Orthogonal Frequency Division Multiple Access) modulated waves, it is effective in realization of high efficiency operation that the power amplifier is designed so as to exhibit its maximum efficiency at a back-off point of 7 dB to 11 dB that is the operating point thereof.
On the other hand, in a power amplifier for use in mobile phone base stations, the transmission signal power level varies from moment to moment depending on a magnitude of communication traffic or the like. A power amplifier is typically designed to exhibit its maximum efficiency during maximum power transmission. However, in this case, the efficiency of the power amplifier is decreased during transmission at a low transmission signal power level.
Thus, under the circumstances there is an increasing demand for reduction of power consumption, it is desired to realize a power amplifier which is capable of operating at higher efficiency and lower power consumption than conventional ones both in a high transmission signal power level region and in a low transmission signal power level region, without depending on the transmission signal power level determined based on a magnitude of communication traffic or the like.
There are known prior art documents relating to this invention. For example, JP-T-2001-518731 (Patent Document 4) discloses an amplifier circuit including a first Doherty amplifier, and a second Doherty amplifier which is cascaded to the first Doherty amplifier.
As described above, there is a demand for realization of a power amplifier capable operating at higher efficiency and lower power consumption than conventional ones both in a high transmission signal power level region and in a low transmission signal power level region without depending on the transmission signal power level determined based on a magnitude of communication traffic or the like.
The aforementioned Patent Document 4 proposes to cascaded two Doherty amplifiers in order to enable the power amplifier to operate efficiently in a wide dynamic range. However, the range of transmission signal power in which the power amplifier is able to operate efficiently cannot be enlarged unless amplification characteristics obtained by the Doherty amplifiers are taken into consideration. Thus, the conventional power amplifiers have a problem that it is difficult to improve the amplification efficiency in a wide transmission signal power range.